Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25-500). It is observed that all the positron parameters are strongly dependent on N when N ≤ 250, and then tend to constant levels when N ≥ 250. The variations of the longest-lived component (I₃, τ₃) show that the size of free-volume holes has a minimum at N = 100 and the hole concentration deereases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I₂, τ₂) with N reflect the changes of defect properties in crystalline regions of PEG.